1. Field of the Invention
The present invention relates to a toner for developing latent electrostatic images, which is utilized in the fields of electrophotography, electrostatic recording and electrostatic printing.
2. Discussion of the Background
Various kinds of electrophotographic methods are conventionally known as disclosed in U.S. Pat. No. 2,297,691 and Japanese Patent Publications 42-23910 and 43-24748. According to these methods, latent electrostatic images are formed on a photoconductor by utilizing a photoconductive material contained in the photoconductor, and the latent electrostatic images are developed into visible images with a toner. The visible toner images thus obtained are transferred to an image-receiving medium such as a sheet of paper when necessary. The transferred toner images are then fixed on the image-receiving medium, for instance, with the application of heat and/or pressure thereto, or by the application of a solvent vapor thereto. Toner-image-bearing copies are thus obtained.
Many systems for fixing the toner image on the image-receiving medium have been developed. Among them, the pressure and heat application system, that is, the heat roller fixing is generally employed at the present stage. A heat roller for use with the heat roller fixing method is fabricated in such a manner that the surface of the heat roller is coated with a material having releasability with respect to the toner to be employed. The image-receiving medium which bears the toner image thereon is caused to pass through the heat roller in such a condition that the toner-image-bearing surface of the image-receiving medium is brought into contact with the surface of the heat roller with the image-receiving medium being urged to the heat roller by the application of pressure, thereby achieving the fixing of toner image on the image-receiving medium.
The image-fixing at a low temperature, which has become an important research topic, can be attained by decreasing the melt viscosity of a binder resin for use in the toner. More specifically, there is proposed a method of decreasing the molecular weight and the glass transition temperature of the binder resin. However, the decrease of the molecular weight and the glass transition temperature of the binder resin means the deterioration of the preservability of the toner. Toner particles tend to cause the blocking phenomenon, and the toner particles are fused and attached to the surface of a development drum.
In Japanese Patent Publication 63-32182 it is proposed to attain the image-fixing at a low temperature and improve the fluidity of toner particles and prevent the stain of a toner-bearing member such as a photoconductor with the toner particles. In the above-mentioned proposal, the binder resin for use in the toner comprises a mixture of a low-molecular weight vinyl polymer which shows at least one maximum peak in the specified low molecular weight region and a high-molecular weight vinyl polymer which shows at least one maximum peak in the specified high molecular weight region. The amount of the low-molecular weight vinyl polymer is relatively large in the toner to improve the image-fixing properties. As a result of the study by the inventors of the present invention, however, it is confirmed that there are too many vinyl polymer components which cannot effectively contribute to the improvement of the image-fixing properties between the maximum peak of the low-molecular weight vinyl polymer component and that of the high-molecular weight vinyl polymer component. Therefore, not only the image-fixing properties, but also the off-set resistance is still insufficient for practical use.
Understandably, therefore, it is extremely difficult to attain the fixing of toner images at a low temperature, expand the temperature region in which the toner images can be fixed on the image-receiving medium, and satisfy the general requirements for the toner, such as the preservability, fluidity and durability, at the same time.
To solve the above-mentioned problems, a resin composition for use in the toner is proposed in Japanese Laid-Open Patent Application 5-331374. The resin composition comprises resin particles, each resin particle comprising a matrix and domain particles with an average particle diameter of 5 .mu.m or less, which are dispersed in the matrix. The glass transition temperature Tg.sub.1 of a resin P.sub.1 for use in the domain particles is in the range from 0.degree. to 60.degree. C., and the glass transition temperature Tg.sub.2 of a resin P.sub.2 for use in the matrix is in the range from 40.degree. to 90.degree. C., and the relationship of Tg.sub.2 .gtoreq.Tg.sub.1 +10.degree. C. is satisfied. The resin P.sub.2 comprises a carboxyl group, while the resin P.sub.1 comprises no carboxyl group. In addition, according to the measurement by gel permeation chromatography (GPC) the molecular weight of the resin P.sub.2 is in the range from 5,000 to 20,000, and that of the resin P.sub.1 is in the range from 500,000 to 1,000,000, with at least one maximum peak in the respective molecular weight regions. The maximum peak in the low molecular weight region has a height H.sub.1, the maximum peak in the high molecular weight region has a height H.sub.3, and the minimum peak between the both maximum peaks has a height H.sub.2, with the ratio of H.sub.1 :H.sub.2 :H.sub.3 being (3-25):1:(1.5-12). Furthermore, the aforementioned resin composition for use in the toner is characterized in that the ration of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of the resin composition is 15 to 80.
The image-fixing at a low temperature can be achieved and the blocking resistance of the toner is improved by using the above-mentioned toner composition. However, there is the problem that a so-called spent toner is deposited on the carrier. The reason for this problem is that the domain particles for use in the resin composition cannot be uniformly dispersed in the matrix and the domain particles with a desired particle diameter cannot be obtained. When the domain particles are not uniformly dispersed in the matrix, many domain particles whose melting point is lower than that of the matrix locally stud the surface of the matrix. Consequently, the spent-toner preventing effect inherent in the matrix is disturbed by the domain particles which are present on the surface of the matrix, and the domain particles are easily attached to the carrier.
As previously explained, when each resin particle of a binder resin for use in the toner comprises a matrix and domain particles dispersed in the matrix, it is essential that the domain particles with a predetermined particle diameter be uniformly dispersed in the matrix in order to achieve the fixing of a toner image at a low temperature.
The resin composition as disclosed in Japanese Laid-Open Patent Application 5-331374 is prepared in such a manner that two kinds of resins which are separately obtained by polymerization are dissolved and stirred in a polar or non-polar solvent with the application of heat thereto. Therefore, it is difficult to obtain the domain particles with a desired particle diameter, and uniformly disperse the domain particles in the matrix. In particular, when a vinyl resin is used for the domain particles, the above-mentioned problems are conspicuous, and the amount of the spent toner deposited on the carrier is increasing as the image-fixing temperature is decreasing.
Furthermore, there is also the problem that the charge quantity of the toner changes depending upon the ambient conditions. When image formation is carried out under the circumstances of high temperature and high humidity, the image density of the obtained toner image is lowered because of the decrease of the charge quantity of the toner.
There is no toner that can satisfy all the above-mentioned mentioned requirements at the present stage.